1. Field of the Invention
The present invention relates to a negative pressure control apparatus for a brake booster, and particularly to a negative pressure control apparatus for controlling negative pressure of a brake booster which is actuated by using intake manifold negative pressure as a power source.
2. Description of the Related Art
Conventionally, as disclosed in Japanese Laid-Open Patent Application No. 8-164840, a negative pressure control apparatus for controlling negative pressure of a brake booster is known. The brake booster is a mechanism which assists a brake operation by using an intake manifold negative pressure as a power source so that a larger braking force can be generated.
The above-mentioned negative pressure control apparatus is applied to an engine which has a fuel injector disposed inside a combustion chamber and directly injects fuel into the combustion chamber (hereinafter such an engine is referred to as a direct injection engine). According to the direct injection engine, fuel economy can be improved by fully opening a throttle valve so that a large volume of air is supplied to the combustion chamber to achieve stratified charge combustion when, for example, the engine is operating under a low load. However, in the direct injection engine, sufficient negative pressure may not be supplied to the brake booster since the intake manifold negative pressure is decreased by the throttle valve being fully opened even when an accelerator pedal is not being operated. For this reason, the above-mentioned negative pressure control apparatus generates a larger intake manifold negative pressure by forcibly closing the throttle valve when the negative pressure of the brake booster becomes smaller than a predetermined value.
When the throttle valve is forcibly closed, the operating state of the engine is changed, which may affect driving performance of a vehicle. Therefore, the process for generating a larger intake manifold negative pressure should be performed only as necessary. Additionally, negative pressure which is required in the brake booster changes in accordance with situations. For example, when both a brake pedal and an accelerator pedal are depressed, a larger negative pressure is required so as to generate a larger braking force by a value matching the driving force generated by the depression of the accelerator pedal.
However, the above-mentioned conventional negative pressure control apparatus simply closes the throttle valve when the negative pressure of the brake booster becomes smaller than the predetermined value. Thus, in the conventional apparatus, the throttle valve may be unnecessarily closed and thus generate unnecessary negative pressure, or sufficient negative pressure may not be maintained in the brake booster resulting in an insufficient assist force, depending on a setting of the predetermined value.